Modern herbicides are highly effective tools, however, expectations for performance often exceed what is observed in the field. The grower, and in many situations the dealer and/or custom applicator, should assume a major portion of the responsibility for product performance. Unfortunately, aggressive marketing strategies by many herbicide manufacturers often shift responsibility from the grower to the manufacturer. This has relieved the grower from using best management practices with herbicides, which may result in reduced performance and the need for additional herbicide inputs.

Successful herbicidal control is dependent upon several factors including:

• selection of the appropriate product and rate,
• optimum timing of application,
• uniform coverage of the target, and
• use of supplemental strategies when required.

A recent paper in Weed Science illustrates the importance of management in getting the most out of herbicides. The paper summarized research conducted in five Midwest states evaluating the performance of Accent for postemergence grass control in corn. The data presented in the accompanying table show the average of nine experiments conducted during 1992 and 1993.

A standard preemergence (PRE) treatment was included in the experiment. The PRE treatment provided 78 percent control of giant foxtail, and corn yields were lower than many of the post treatments due to inadequate control. The PRE treatments were applied within four days of planting; foxtail control was reduced at locations with insufficient rain after application to move the herbicide into the soil profile.

Combinations of Accent and cultivation, or late applications of Accent (when foxtail was taller than 4 inches) provided 90 percent or better foxtail control. Although the late Accent treatments provided acceptable foxtail control, corn yields were significantly lower than other treatments, presumably due to early season competition prior to the postemergence application. The treatment that provided the best foxtail control and highest yield was the early Accent application (foxtail at 1- to 2-inch height) followed by cultivation. The early application without cultivation provided reduced foxtail control due to late emerging weeds, and lower corn yields than the same herbicide treatment with cultivation.

This research reinforces several important points concerning weed management. First, single strategy programs are unlikely to provide consistent weed control and optimum yields. Second, relatively small differences in application timing can have a significant impact on herbicide performance and crop yields. And finally, cultivation remains an effective and important management strategy.

Growers must develop realistic expectations for herbicide performance so they can design a weed management program with a high likelihood of success. Unrealistic expectations frequently lead to inefficient use of herbicides and, therefore, a greater probability of failure.

Effect of grass size and Accent rate on giant foxtail and woolly cupgrass control and corn yields.¹

¹ Data are means of nine experiments conducted in five states during 1992 and 1993.

² Means within columns followed by the same letter are not different at P<0.05.

Source: Tapia et al (1997), "Postemergence herbicide application timing effects on annual grass control and corn grain yield," Weed Science 45:138-143.